The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
The present disclosure relates generally to a seismic source, system, and method for performing acoustic measurement and analysis of subsurface geological strata and layers. More specifically, some embodiments disclosed herein may be directed to a seismic source, system and method used for placing seismic devices, such as a seismic source and a seismic receiver, downhole in a well. The seismic devices may then be used to identify characteristics of subsurface layers including such features as oil and gas deposits located therein, for example.
In a typical seismic survey, seismic signals are generated by producing seismic waves. The seismic waves may be reflected off of various subsurface features due to the contrasting acoustical impedance present at stratum boundaries. These reflected seismic waves are collected as seismic data and are analyzed to provide a representational image of a subsurface structure or feature. As an example, in some systems a vibrator, for generating seismic waves, and one or more geophones, used for receiving reflected seismic waves, may be located on the ground surface.
However, it is known that there is a low velocity surface layer near the ground surface that attenuates portions of the generated seismic waves. In a typical ground surface survey, the generated waves must pass through this attenuation layer, be reflected off of the subsurface feature located thousands of meters below the surface in some cases, and then the reflected seismic waves must pass through the attenuation layer a second time prior to being received by the ground based receivers.
The low velocity surface layer attenuates high frequency seismic waves. However, high frequency seismic waves also have the ability to propagate relatively long distances through the ground. The low velocity surface layer has a greater impact on the portion of the generated seismic wave that has the best chance of reaching a subsurface feature located deep underground.
Any high frequency seismic waves that do reach the subsurface feature and are reflected back towards the ground surface have to pass again through the low velocity subsurface layer. This reduces the ability of the high frequency portion of the reflected seismic wave from eventually reaching a geophone (or other seismic receiver) located on the ground surface.
Consequently, the reflected seismic waves detected by a ground based geophone during a survey of a subsurface target have frequencies that are less likely to be attenuated. In some cases, the frequency range of reflected seismic waves is located, for example, in a range of frequencies between 10 Hz and 60 Hz. Higher frequency waves would be able to go farther and provide a more precise picture of the subsurface characteristics.
One alternative is to use seismic waves generated by a seismic source, such as detonating dynamite for example, below the low velocity surface layer. Generated seismic waves would then pass through the low velocity surface layer only once before being detected by a geophone placed on the ground surface. High-frequency seismic waves would be less likely to be attenuated as compared with a case where the generated seismic waves pass through the low velocity surface layer twice. Accordingly, the geophones may capture slightly higher-frequency reflected seismic waves.
Taking this idea one step further, when the receiver is placed below the surface in a well, seismic waves never pass through the low velocity surface layer before being detected by the in well geophones. Then, high-frequency seismic waves are less likely to be attenuated as compared with the case where generated and reflected seismic waves must pass through the low velocity surface layer twice. In such cases, the in well geophones may capture even higher-frequency reflected seismic waves.
In addition, since a propagation path of seismic waves from the point of generating seismic waves to the reflected surface and back to the in well geophones is shortened, the in well geophones may capture greater amounts of higher-frequency seismic waves, increasing the precision and analysis of the subsurface features.
However, to explode dynamite below the low velocity surface layer, it is required to drill a hole for each time of explosion. Also, since the propagation path of seismic waves generated by a dynamite explosion is in the limited range of 50 m to 100 m, high-frequency seismic waves are more likely to be attenuated. Thus, a high resolution image is less likely to be obtained.